Wind-vector satellite radiometer

The theory of microwave emission from a wind-roughened water surface predicts that the surface brightness temperature (TB) depends on the relative wind direction φ_r (i.e., the direction of the wind relative to the azimuth angle of the observation). Satellite observations from the microwave radiometer SSM/I are used to investigate this wind direction effect. The SSM/I observations are collocated with 40,000 buoy reports that give wind speed and direction. These data show that the v-pol and h-pol brightness temperatures vary harmonically with φ_r: T_B=T_Bo+B₁cos φ _r+B₂cos 2φ_r. The amplitudes of the variation, B₁ and B₂, depend on the wind speed, incidence angle, frequency, and polarization. This T_B Variation with wind direction provides the means to estimate both wind speed and direction from microwave radiometer observations. Monthly global wind vector maps are computed from the SSM/I observations. These maps look reasonable and show the large-scale features of global circulation over the oceans. Computer simulations are done for satellite microwave radiometer that looks both forward and aft. The simulations indicate that a two-look radiometer can measure wind direction to an accuracy of 20° or better. The simulations also indicate that multiple wind direction solutions (i.e., `aliases´ or `ambiguities´) are less problematic for radiometers as compared to scatterometers. It is proposed that a two-look radiometer that measures the first three Stokes parameters be flown on future satellites